Systems for converting cinematographic (i.e., motion picture) images into digital signals typically include a film transport which moves film from a supply reel to a take up reel. Between the reels, the film passes through a scan gate or shuttle where the image is captured by an image sensor, which is typically a charge coupled device ("CCD") sensor. Such systems are known in the motion picture industry as CCD based telecines or scanners. In these systems, it is important that each scanned frame be positioned in the same location for scanning as its neighboring frames. If this is not possible, the position of each frame during scanning must be known so that any position variation can be corrected in the digital signal, such as by digital image manipulation techniques. In either event, the digital signal corresponds to a "steady" image sequence. Various special effects can then be applied to the digital signals representing such steady image sequences.
To achieve steadiness, film scanners have traditionally used mechanical registration devices in an attempt to duplicate the manner in which a motion picture camera which exposed the film has pin registered each image frame. Such mechanical registration devices range from the most basic film edge guides, through the use of sprocket wheels, to full pin registration systems. Film edge guides attempt to provide control over lateral motion of the film, but edge guides do a poor job of duplicating the registration provided by a pin registered camera. Pin registration systems attempt to duplicate the pin registration system of the exposing camera, in which the pins fully engage through perforation holes in the film. However, sprocket wheels using pins which partially fit a film perforation, reference only one edge of the perforation, while the typical tolerance on the dimension between the center of a film perforation and the film edge varies by +/-50 micrometers. Additionally, the standard specification for the variation of the width of a film perforation is +/-10 micrometers. Thus, such partially fitting pins leave room for mis-positioning of a perforation, and hence, an associated film frame during scanning. While these numbers may seem small, it must be remembered that following image processing, such as for special effects, the scanned motion picture images will be re-printed on a motion picture film. This re-printed film (or copies of it) will be substantially magnified for projection onto the screen of a theater and the resulting film will be magnified for projection. Mis-positioning between successive frames during scanning thereby results in a highly visible and undesirable movement of an image or portion of a composite image on a screen.
Attempts have been described in U.S. Pat. No. 5,266,979 to combine edge guidance and full fitting pins with a sprocket wheel arrangement. However, this system over constrains the film as it passes through a film gate for scanning, which can result in film flatness or image distortion problems. Further, in any sprocket wheel arrangement, since the same pins do not contact each frame, a large emphasis is placed upon the quality of manufacturing processes for sprocket wheels.
Pure pin registration systems utilize both a full fitting big pin to engage perforations along one side of the film, and a partial fitting little pin to engage perforations along the other edge of the film, to exactly constrain the film in the x and y directions, and to prevent rotation of the film. The same pins are used to register each frame. When properly designed and maintained, these systems provide the best duplication of camera pin registration. However, there are disadvantages to the use of mechanical pin registration. In general, such systems are intermittent motion systems with lower throughput. Further, the design, construction, and maintenance of the registration mechanism and the pins are expensive. In addition, the complexity and cost of the subsystems surrounding a pin registered film scanning gate is relatively high. Furthermore, the risk of perforation damage is a constant concern. This is particularly true since film age and means of storage will effect the dimensions and elasticity of the film. Thus, the risk of damage to archived films increases in a pin registered system. As a result, pin registered systems are not used in high throughput motion picture film scanning systems.
Non-contact perforation detection systems avoid the risk of film damage, and can run at high speed. In addition, non-contact perforation detection systems are inexpensive and can be easily maintained. Several non-contact perforation detection systems have been described, for example, in commonly-assigned U.S. Pat. No. 5,107,127, the disclosure of which is herein incorporated by reference. However, previous non-contact perforation detection systems do not appreciate any problems associated with dimensional variations of a film being scanned. Furthermore, such systems typically transport the film in a continuous fashion and detect the perforation at a location remote to the scanning position. Because it is difficult to accurately control film motion without any slip or film dimensional variations, such systems result in less than optimal steadiness.